Organic matter accumulation of the Upper Triassic Ma'antang shales in the Longmenshan Foreland Basin (western Sichuan, China)

Abstract

The Upper Triassic Ma'antang Formation is not only an important set of source rocks in the Sichuan Basin but may also be a successor for shale gas in China. In this study, the provenance, tectonic setting, chemical weathering, environment and productivity of the middle–late Carnian (Tuvalian) are reconstructed from petrological and geochemical data to determine the origin of the Ma'antang shales. The brachiopod-dominated fossil composition, type II-dominated organic matter, and fluctuating water salinity reflect that the shale formed in a marine environment influenced by terrestrial influx. TiO2 vs. Zr and La/Th vs. Hf diagrams, as well as the combination of major elements indicate that the Sichuan Basin was a collisional setting in the Tuvalian, and the provenance was felsic igneous source rocks. Redox-sensitive elements indicate dysoxic–anoxic conditions. Samples with Cd/Mo < 0.1 and Co*Mn > 0.4 show normal levels of Cd and relative enrichment of Mn and Co, which supports that the shale is more likely to be a product of anoxic preservation mode associated with the restricted basin. In contrast, the moderate productivity conditions reflected by Babio and P/Ti are also positive factors for organic matter accumulation. Based on the reconstruction of these environmental parameters, several driving factors that promote the formation of high TOC shales are explored. It is believed that the rapid tectonic subsidence in the early stage of foreland basin evolution (undercompensation period) created accommodation space for the shale, and the deep water also promoted anoxic bottom water, which facilitated the preservation of organic matter. The Carnian Pluvial Episode (CPE) increased global weathering and rainfall, allowing the oceans to receive more terrestrial nutrients, and freshwater influx caused stratification due to differences in salinity between the surface water and bottom water. Volcanic ash from the Wrangellia Large Igneous Province (W-LIP) and volcanic ash from local volcanoes were transported over long distances in the Late Triassic megamonsoons. Both were able to stimulate marine productivity, maintain and expand oceanic anoxia, and were beneficial for organic matter accumulation.